Modular multilevel converter (MMC) has been a competitive candidate for voltage source converter (VSC) based high voltage direct current (HVDC) transmission due to its advantages of avoiding IGBT series operation, improved scalability, high efficiency and excellent output waveforms with low harmonic distortion.
At present, most of the VSC based HVDC projects adopt the half-bridge sub-module (HBSM) based MMC which cannot isolate a DC short fault and requires breakers on AC side or DC side for fast fault isolation. However, breakers on AC side can only clear the DC fault at the zero crossing point of AC-side current meaning considerable delay time existing before the breaker acts, and the DC short circuit fault can be turned into an AC short circuit fault. In addition, the reclosing timing sequence for an AC breaker is comparatively complex. The technique of DC-side breaker is not mature and highly cost-consuming due to no zero crossing in the DC current. According to CIGRE reports, development of DC-side breakers for the industrial application of ±500 kV and ±800 kV need at least 10 years and 15 years, respectively. A full-bridge sub-module (FBSM) based MMC can output negative voltage, and thus being capable of isolating DC short-circuit faults actively. At present, the FBSM based MMC works in the HBSM based MMC mode during normal operation meaning FBSMs are controlled to only output positive or zero voltage. However, the number of power devices is doubled in each FBSM compared to the HBSM, resulting in increased cost and power losses, thus it is disadvantageous to FBSM based MMC's industrial application.